Grant Williams started and played 25 minutes for Boston in its 116-108 victory over No. 3 Milwaukee at Fiserv Forum.
No. 2 seed Boston has tied the Eastern Conference semifinals, 2-2.
Williams recorded nine points and four rebounds for the Celtics in his first postseason start.
Williams was 3-for-9 from the floor against the Bucks. He was 1-for-5 from beyond the 3-point arc and 2-for-2 from the free throw line.
The Eastern Conference semifinals series resumes with game No. 5 in Boston Wednesday.
Tipoff is slated for 7 p.m. EDT at TD Garden. TNT will televise the contest.
Williams was a first round draft pick by the Celtics in 2019.